During the manufacture of the thin film transistor liquid crystal display, the homogeneity of the liquid crystal orientation directly affects various qualities of the display. In manufacture, the directional alignment of the liquid crystal molecules is usually achieved by coating an alignment film on the substrate. As shown in FIG. 1, the alignment film 2 coated on the surface of the substrate 1 is directionally aligned along the rubbing direction under the action of the rapidly rotating rubbing roller 3, and after the assembly of the thin film transistor (TFT) array substrate with the color filter (CF) substrate, the liquid crystal molecules are aligned along a certain direction through the anchoring effect of the alignment film on the surface liquid crystal molecules. The major component of the alignment film is polyimide. With the continuously elevating requirements on the display quality of the display, especially in high end display mode such as in plane switching (IPS), fringe field switching (FFS) and the like, the homogeneity of the directional alignment of the liquid crystal molecules is paid more and more attention. Therefore, the ability of the alignment film to pose a directional effect on the liquid crystal molecules also becomes a focus.
The traditional polyimide alignment film primarily involves the polymerization of pyromellitic dianhydride and p-phenylenediamine in a solvent (such as N-methylpyrrolidone (NMP)) to generate a polyamic acid and heat dehydration of the polyamic acid to form a polyimide alignment film (as shown in FIG. 2). By employing the aforementioned method, an improved polyimide alignment film can be obtained by the polymerization of the derivatives, analogs or modified species with side chain substitution of pyromellitic dianhydride and p-phenylenediamine. The polyimide alignment film obtained by this method has a structure of long, single and straight chains, and between the chains, stacking is formed through binding by intermolecular interaction such as π-π interaction, hydrogen bond, or spatial hindrance. As shown in FIG. 3, the liquid crystal molecule 5 is adsorbed onto the polyimide. The intermolecular interaction between the liquid crystal molecules and the polyimide determines the alignment of the liquid crystal molecules in the polyimide surface layer. Research shows that the shape of the backbone of the polyimide, the attractive effect between the aromatic rings on the backbone and the aromatic rings on the liquid crystal molecules, and the repulsive effect of the side chain alkyl on the liquid crystal molecules dominate the directional alignment of the liquid crystal molecules. In prior art, the directional force of the alignment film on the liquid crystal molecules is often strengthened by increasing the proportion or volume of the aromatic component in the polyimide backbone or embedding a long alkyl side chain. Since the traditional polyimde backbone is not completely a structure of a straight chain, enhancing the directional effect of the alignment film on the liquid crystal molecules by simply increasing the proportion or volume of the aromatic component will cause too strong π-π interaction between the chains in the polyimide which causes the backbones of the polyimide to bind each other, thereby causing the liquid crystal molecules adsorbed onto the surface of the polyimide alignment film to have different twisting angles in different regions of the polyimide back bone, which in turn causes the inconsistency among the alignment directions of the liquid crystal molecules 5 in different regions of the polyimide backbone 4 (resulting in the situation as shown in FIG. 3). This will prevent the liquid crystal molecules from aligning in a good directional and consistent manner, cause the decrease of the contrast of the liquid crystal display, and thus cause the drop of the display quality of the liquid crystal display.